Downwelling 2-pi scalar irradiance as energy of electromagnetic radiation (PAR wavelengths) in the water body by 2-pi scalar radiometer

OXYSSC01

1

Percent

BK_SBE43

Saturation of oxygen {O2 CAS 7782-44-7} in the water body [dissolved plus reactive particulate phase] by Sea-Bird SBE 43 sensor and calibration against sample data and computation from concentration using Benson and Krause algorithm

POTMCV01

1

Degrees Celsius

WC_Potemp

Potential temperature of the water body by computation using UNESCO 1983 algorithm

PRESPR01

1

Decibars

Pres_Z

Pressure (spatial co-ordinate) exerted by the water body by profiling pressure sensor and corrected to read zero at sea level

PSALCC01

1

Dimensionless

P_sal_CTD_calib

Practical salinity of the water body by CTD and computation using UNESCO 1983 algorithm and calibration against independent measurements

SIGTPR01

1

Kilograms per cubic metre

SigTheta

Sigma-theta of the water body by CTD and computation from salinity and potential temperature using UNESCO algorithm

TEMPCU01

1

Degrees Celsius

Uncal_CTD_Temp

Temperature of the water body by CTD and NO verification against independent measurements

TURBPR01

1

Nephelometric Turbidity Units

Turb

Turbidity of the water body by in-situ optical backscatter measurement and laboratory calibration against formazin

UWIRPP01

1

Watts per square metre

ScalarUwirPAR

Upwelling 2-pi scalar irradiance as energy of electromagnetic radiation (PAR wavelengths) in the water body by 2-pi scalar radiometer

Definition of Rank

Rank 1 is a one-dimensional parameter

Rank 2 is a two-dimensional parameter

Rank 0 is a one-dimensional parameter describing the second dimension of a two-dimensional parameter (e.g. bin depths for moored ADCP data)

Problem Report

Beam attenuance

The transmissometer appeared to suffer from slight pressure or temperature hysteresis at depth for casts from 19 to 46. The pattern showed a minimum attenuance in the range 200-300 db and then a slight increase in value as the profile went deeper. Casts 26 and 40 appeared to be obviously affected, and users should take account of quality control flags. For casts 26 and 40 where data are binned to 1 decibar, there will be large sections of the cast where the data are null, due to the absence of good quality data for each bin.

The transmissometer has been calibrated with pure water as the reference for 100% transmission and therefore beam attenuation values in clear water should be close to 0 m-1. Chelsea Instruments advise that ALPHAtracka is calibrated at the factory at 20°C in distilled water with an electrical conductivity less than one µS cm-1 and filtered to better than 5 µm and that it is possible that the user will encounter water which is purer than that used during the calibration. Indeed the minimum attenuance values for the profiles from the stainless steel rig mounted tranmissometer were lower then 0 m-1, suggesting that the calibration procedure recommended by Sea-Bird and Chelsea Instruments may need adjusting to use deep clear oceanic water as the reference for 100% transmission. The attenuance data from the transmissometer will need further offset correction to bring them in line with recognised values. Whether this should be done for the dataset as a whole or on a cast by cast basis is for the user to decide based on their requirements. The absolute attenuation values are therefore questionable but the relative profile should be reliable except for profiles where hysteresis was a problem at depth.

AMT16 Data Quality Report

Attenuance

Due to the transmissometer calibration issues, many of the attenuance values were negative (beyond the range of the parameter). All negative values were flagged 'M'. This does not necessarily mean that the data are scientifically useless, just that the calibration coefficients may be slightly out. Where previous 'T' flags were overwritten, the original flagged data are available on request. The transmissometer was removed from the titanium rig on Julian Day 145 because it was providing unreliable data up until that point, so the attenuance channel has been removed from casts 2T, 4T, 5T, 7T, 9T and 12T.

The nominal chlorophyll-a values have been calculated from the CTG Aquatracka MKIII fluorometer data (with manufacturer's calibration applied) from the up-cast at bottle firing and the fluorometric chlorophyll-a data from sampled bottles. The calibrations were split between the two fluorometers used on the different CTD rigs. Where samples were not supplied or too few to generate a calibration and could not be grouped with other casts, the fluorometer profiles have not been calibrated. The sampling strategy for the extracted chlorophyll-a dataset used to calibrate the fluorometer focused on the upper water column, therefore the calibration is biased towards these depths. The calibration may not be as reliable below depths ~150 m. Casts 1S, 2T, 61T, 63T, 64T, 65T and 67T have not been calibrated. The extracted chlorophyll-a dataset is available for users to derive their own calibrations should they wish.

Downwelling and upwelling sub-surface PAR irradiance

For downwelling PAR, some data points were beyond the maximum range of the parameter and so were flagged as suspect. Some data points in the upwelling PAR channel were below the minimum range of the parameter so these data points were also flagged as suspect. The PAR channels in cast 2T are constant. As this was not a deep cast past 1000 m, the sensors would not have been removed. It is likely that the sensor caps were not removed prior to the cast, as the time of the cast was solar noon.

Turner Designs Cyclops 7 fluorometer voltage (FVLTPELN)

No calibration details were provided so only the raw voltage is available. For many of the casts a constant voltage of zero was returned but these have been left in for comparison with casts where a signal was returned.

Open Data supplied by Natural Environment Research Council (NERC)

Sea-Bird Dissolved Oxygen Sensor SBE 43 and SBE 43F

The SBE 43 is a dissolved oxygen sensor designed for marine applications. It incorporates a high-performance Clark polarographic membrane with a pump that continuously plumbs water through it, preventing algal growth and the development of anoxic conditions when the sensor is taking measurements.

Two configurations are available: SBE 43 produces a voltage output and can be incorporated with any Sea-Bird CTD that accepts input from a 0-5 volt auxiliary sensor, while the SBE 43F produces a frequency output and can be integrated with an SBE 52-MP (Moored Profiler CTD) or used for OEM applications. The specifications below are common to both.

Discovery Cruise AMT16 CTD Instrumentation for the titanium frame.

Two different CTD frames were used - a stainless steel frame and a titanium frame used for trace metal sampling. This document describes the instrumentation on the titanium frame.

Titanium

The CTD unit was a Sea-Bird Electronics 911 plus system, with dissolved oxygen sensor. The CTD was fitted with a transmissometer and a fluorometer. All instruments were attached to a Sea-Bird SBE 32 carousel (titanium). The table below lists more detailed information about the various sensors.

Sensor

Model

Serial Number

Calibration

Comments

Pressure transducer

Digiquartz temperature compensated pressure sensor

79501

23/09/2003

-

Conductivity sensor 1

SBE 4C

2851

15/02/2005

-

Conductivity sensor 2

SBE 4C

2858

16/02/2005

-

Temperature sensor 1

SBE 3P

4380

11/02/2005

-

Temperature sensor 2

SBE 3P

4381

11/02/2005

-

Dissolved oxygen

SBE 43

43B-0612

19/01/2005

-

Fluorometer

Chelsea MkIII Aquatracka

88/2960/163

13/11/2002

-

PAR sensor - upwelling

Chelsea PAR sensor

04

01/09/2004

Added to the rig on 25/05/2005

PAR sensor - downwelling

Chelsea PAR sensor

02

01/09/2004

Added to the rig on 25/05/2005

Light scatter sensor

Sea Tech Light scatter sensor

338

16/04/1997

-

Transmissometer

Chelsea MkII Alphatracka

161049

03/05/2001

0.25 m path

Change of sensors during cruise: PAR sensors were removed from the rig for casts to depths greater than 500m. Chelsea Mk II Alphatracka 161049 was removed from the rig on 25/05/2005.

Sampling device

Sea-Bird Electronics SBE 911 and SBE 917 series CTD profilers

The SBE 911 and SBE 917 series of conductivity-temperature-depth (CTD) units are used to collect hydrographic profiles, including temperature, conductivity and pressure as standard. Each profiler consists of an underwater unit and deck unit or SEARAM. Auxiliary sensors, such as fluorometers, dissolved oxygen sensors and transmissometers, and carousel water samplers are commonly added to the underwater unit.

Underwater unit

The CTD underwater unit (SBE 9 or SBE 9 plus ) comprises a protective cage (usually with a carousel water sampler), including a main pressure housing containing power supplies, acquisition electronics, telemetry circuitry, and a suite of modular sensors. The original SBE 9 incorporated Sea-Bird's standard modular SBE 3 temperature sensor and SBE 4 conductivity sensor, and a Paroscientific Digiquartz pressure sensor. The conductivity cell was connected to a pump-fed plastic tubing circuit that could include auxiliary sensors. Each SBE 9 unit was custom built to individual specification. The SBE 9 was replaced in 1997 by an off-the-shelf version, termed the SBE 9 plus , that incorporated the SBE 3 plus (or SBE 3P) temperature sensor, SBE 4C conductivity sensor and a Paroscientific Digiquartz pressure sensor. Sensors could be connected to a pump-fed plastic tubing circuit or stand-alone.

Temperature, conductivity and pressure sensors

The conductivity, temperature, and pressure sensors supplied with Sea-Bird CTD systems have outputs in the form of variable frequencies, which are measured using high-speed parallel counters. The resulting count totals are converted to numeric representations of the original frequencies, which bear a direct relationship to temperature, conductivity or pressure. Sampling frequencies for these sensors are typically set at 24 Hz.

The temperature sensing element is a glass-coated thermistor bead, pressure-protected inside a stainless steel tube, while the conductivity sensing element is a cylindrical, flow-through, borosilicate glass cell with three internal platinum electrodes. Thermistor resistance or conductivity cell resistance, respectively, is the controlling element in an optimized Wien Bridge oscillator circuit, which produces a frequency output that can be converted to a temperature or conductivity reading. These sensors are available with depth ratings of 6800 m (aluminium housing) or 10500 m (titanium housing). The Paroscientific Digiquartz pressure sensor comprises a quartz crystal resonator that responds to pressure-induced stress, and temperature is measured for thermal compensation of the calculated pressure.

Additional sensors

Optional sensors for dissolved oxygen, pH, light transmission, fluorescence and others do not require the very high levels of resolution needed in the primary CTD channels, nor do these sensors generally offer variable frequency outputs. Accordingly, signals from the auxiliary sensors are acquired using a conventional voltage-input multiplexed A/D converter (optional). Some Sea-Bird CTDs use a strain gauge pressure sensor (Senso-Metrics) in which case their pressure output data is in the same form as that from the auxiliary sensors as described above.

Deck unit or SEARAM

Each underwater unit is connected to a power supply and data logging system: the SBE 11 (or SBE 11 plus ) deck unit allows real-time interfacing between the deck and the underwater unit via a conductive wire, while the submersible SBE 17 (or SBE 17 plus ) SEARAM plugs directly into the underwater unit and data are downloaded on recovery of the CTD. The combination of SBE 9 and SBE 17 or SBE 11 are termed SBE 917 or SBE 911, respectively, while the combinations of SBE 9 plus and SBE 17 plus or SBE 11 plus are termed SBE 917 plus or SBE 911 plus .

Chelsea Technologies Group Aquatracka MKIII fluorometer

The Chelsea Technologies Group Aquatracka MKIII is a logarithmic response fluorometer. Filters are available to enable the instrument to measure chlorophyll, rhodamine, fluorescein and turbidity.

It uses a pulsed (5.5 Hz) xenon light source discharging along two signal paths to eliminate variations in the flashlamp intensity. The reference path measures the intensity of the light source whilst the signal path measures the intensity of the light emitted from the specimen under test. The reference signal and the emitted light signals are then applied to a ratiometric circuit. In this circuit, the ratio of returned signal to reference signal is computed and scaled logarithmically to achieve a wide dynamic range. The logarithmic conversion accuracy is maintained at better than one percent of the reading over the full output range of the instrument.

Two variants of the instrument are available, both manufactured in titanium, capable of operating in depths from shallow water down to 2000 m and 6000 m respectively. The optical characteristics of the instrument in its different detection modes are visible below:

Excitation

Chlorophyll a

Rhodamine

Fluorescein

Turbidity

Wavelength (nm)

430

500

485

440 *

Bandwidth (nm)

105

70

22

80 *

Emission

Chlorophyll a

Rhodamine

Fluorescein

Turbidity

Wavelength (nm)

685

590

530

440 *

Bandwidth (nm)

30

45

30

80 *

* The wavelengths for the turbidity filters are customer selectable but must be in the range 400 to 700 nm. The same wavelength is used in the excitation path and the emission path.

The instrument measures chlorophyll a, rhodamine and fluorescein with a concentration range of 0.01 µg l -1 to 100 µg l -1 . The concentration range for turbidity is 0.01 to 100 FTU (other wavelengths are available on request).

The instrument accuracy is ± 0.02 µg l -1 (or ± 3% of the reading, whichever is greater) for chlorophyll a, rhodamine and fluorescein. The accuracy for turbidity, over a 0 - 10 FTU range, is ± 0.02 FTU (or ± 3% of the reading, whichever is greater).

This sensor was originally designed to assist the study of marine photosynthesis. With the use of logarithmic amplication, the sensor covers a range of 6 orders of magnitude, which avoids setting up the sensor range for the expected signal level for different ambient conditions.

The sensor consists of a hollow PTFE 2-pi collector supported by a clear acetal dome diverting light to a filter and photodiode from which a cosine response is obtained. The sensor can be used in moorings, profiling or deployed in towed vehicles and can measure both upwelling and downwelling light.

Sea Tech Light Back-Scatter sensor

The instrument projects light into the sample volume using two modulated 880 nm Light Emitting Diodes. Light back-scattered from the suspended particles inthe water column is measured with a solar-blind silicon detector. A light stop between the light source and the light detector prevents the measurement of direct transmitted light so that only back-scattered light from suspended particles in water are measured.

The sensor has two ranges permitting the user to measure nearly all suspended particle concentrations found in open ocean or coastal waters. Range for the measurement of suspended particle concentration in water will be approximately 10 mg l -1 if High_Gain is selected. If Low-Gain is selected full scale will be a factor of 3.3 higher or approximately 33 mg l -1 .

Discovery Cruise AMT16 CTD Processing

Sampling strategy

A total of 67 successful CTD casts were made during the cruise, 34 casts used the stainless steel rig and 33 used the titanium rig. Rosette bottles were fired throughout the water column on the upcast of most profiles. Data were measured at 24 Hz by a PC running SEASAVE, Sea-Bird's data acquisition software. The raw data files were supplied to BODC after the cruise.

Originator's processing

Only a subset of files had been partially processed on board during the cruise. The raw data were therefore reprocessed at BODC to produce a homogeneous set of CTD data files for this cruise.

BODC post-processing and screening

BODC used the latest version of the SeaBird Processing software available at the time to process the raw binary data files (DAT files) based on information held in the sensor configuration files (CON files), and bottle firing files (BL).

Sea-Bird processing

The CON files were first checked for any changes which may have occurred during the cruise, none were made. The information was also cross checked against information held in the sensors' calibration reports.

The following SeaBird routines were then carried out using SBE Data Processing software version 5.30a: DATCNV, CELLTM, FILTER, LOOPEDIT, DERIVE, BINAVG, STRIP. After CELLTM was run, tests were carried out to check whether an alignment of the conductivity sensor was necessary. In some instances a lag of 0.007 s was found but since this was not consistent on all casts it was decided that no lag need to be applied to conductivity. A lag of 6 s was applied to the oxygen channel as per SeaBird guidance. Details of the routines and settings used were as follows:

DATCNV converts the raw data into engineering units. Both down and upcasts were selected. All channels were selected for transfer.

The manufacturer's calibration for the fluorometer was applied during Sea-Bird processing as follows:

Stainless steel

Nominal chl-a conc (µg/l) = (0.00948 * 10 voltage ) - 0.0174

Titanium

Nominal chl-a conc (µg/l) = (0.01080 * 10 voltage ) - 0.0270

CELLTM was run on the DATCNV output using SeaBird's recommended settings of alpha= 0.03 and Tau=7.

FILTER was run on pressure using a low pass time constant of 0.15 seconds.

LOOPEDIT was run in order to minimise the marked wake effect linked to ship rolling observed on recent cruises.

DERIVE, BINAVG and STRIP were then run to derive the salinity and oxygen concentration, reduce the data to 2Hz and strip redundant channels from the final sets of ASCII files.

Conversion of transmissometer voltages to beam attenuation

The transmissometer raw voltages have been converted to attenuance values in units of m -1 using manufacturer air/dark/pure water voltages converted to calibration coefficients as per Sea-Bird Application Note No.7 . No air/dark voltages were provided from the cruise so coefficients have been calculated with the most recent dark/air voltages being those provided by the manufacturer.

M = (T w / (W 0 - Y 0 ) * (A 0 - Y 0 ) / (A 1 - Y 1 )

B = -M * Y 1

where

Stainless steel

T w =

% transmission for pure water

100%

W 0 =

voltage output in pure water

4.2220 V

A 0 =

manufacturer's air voltage

4.4045 V

Y 0 =

manufacturer's blocked path voltage

0.0185 V

A 1 =

cruise air voltage

3.918

Y 1 =

cruise blocked path voltage

0.018

The coefficients applied during the cruise and used in BODC processing were M = 21.1680 and B = - 0.3810. However post-cruise calculation of the coefficients from the calibration sheets and cruise blank and air voltages provided to BODC gave values of M = 26.7543 and B = -0.4816. The difference between the coefficients equates to an attenuance offset of -0.9368 m -1 which was applied to the attenuance data.

Conversion of PAR sensor voltages to irradiance

The PAR sensor raw voltages have been converted to PAR irradiance values in units of W m -2 using supplied manufacturer's calibration coefficients.

The instrument configuration was set up with 33 mg/l and a slope of 1. The voltages were converted to Nephelometric Turbidity Units (NTU) with a scale factor of 6.6. This was a relatively new instrument at the time of the cruise and the SeaBird configuration files were yet to be adjusted for correct application of the instrument calibration coefficients. During subsequent transfer and processing the units were reconverted back to a voltage.

Reformatting

The data were converted from Sea-Bird ASCII format into BODC internal format (PXF) using BODC transfer function 357. The following table shows how the variables within the Sea-Bird files were mapped to appropriate BODC parameter codes:

Sea-Bird Parameter Name

Units

Description

BODC Parameter Code

Units

Comments

Pressure, Digiquartz

dbar

CTD pressure

PRESPR01

dbar

-

Temperature [ITS-90]

°C

Temperature of water column by CTD sensor 1

TEMPCU01

°C

-

Temperature, 2 [ITS-90]

°C

Temperature of water column by CTD sensor 2

TEMPCU02

°C

-

Salinity

-

Practical salinity of the water body by CTD sensor 1

PSALCU01

-

-

Salinity, 2

-

Practical salinity of the water body by CTD sensor 2

PSALCU02

-

-

Oxygen

µmol kg -1

Dissolved oxygen concentration

DOXYSU01

µmol l -1

Converted from µmol kg -1 to µmol l -1 using sigma-T during transfer

Fluorescence

mg m -3

Nominal chl-a concentration

CPHLPM01

mg m -3

Manufacturer's calibration applied during processing

Voltage 4

V

Turner Designs Cyclops 7 fluorometer voltage

FVLTPELN

V

Only for stainless steel rig casts No manufacturer's calibration details available

PAR/Irradiance, Biospherical/Licor

W m -2

Downwelling sub-surface PAR irradiance

DWIRPP01

W m -2

Only for titanium rig casts shallower than 500m

PAR/Irradiance, Biospherical/Licor, 2

W m -2

Upwelling sub-surface PAR irradiance

UWIRPP01

W m -2

Only for titanium rig casts shallower than 500m

OBS

NTU

Back-Scatter Sensor voltage

NVLTWR01

V

Only for stainless steel rig casts Converted back to a voltage by dividing by 6.6

OBS

NTU

Turbidity of the water body

TURBPR01

NTU

Only for titanium rig casts

Beam Attenuation

m -1

Beam attenuance

ATTNDR01

m -1

Only for stainless steel rig casts

-

-

Practical salinity of the water body by CTD sensor 1 - sample calibrated

PSALCC01

-

PSALCU01 calibrated against bench salinometer data

-

-

Practical salinity of the water body by CTD sensor 2 - sample calibrated

PSALCC02

-

PSALCU02 calibrated against bench salinometer data

-

-

Dissolved oxygen concentration - sample calibrated

DOXYSC01

µmol l -1

DOXYSU01 calibrated against Winkler titration data

-

-

Fluorometer - sample calibrated

CPHLPS01

mg m -3

CPHLPM01 calibrated against fluorometric chlorophyll-a data

-

-

Oxygen saturation

OXYSSC01

%

Generated by BODC using the Benson and Krause (1984) algorithm wioth parameters DOXYSC01, PSALCC01 and TEMPCU01

UNESCO, 1981. Background papers and supporting data on the International Equation of State of Seawater 1980. UNESCO Technical Papers in Marine Science No. 38, 192pp

Screening

The PXF data were compared with the original data files to ensure that no errors had been introduced during the conversion process. Reformatted CTD data were transferred onto a graphics work station for visualisation using the in-house editor EDSERPLO. Downcasts and upcasts were differentiated and the limits manually flagged. No data values were edited or deleted. Flagging was achieved by modification of the associated BODC quality control flag for suspect or null values.

Salinity and temperature The primary channel should be used in preference to the secondary channel as it has been quality controlled. The problem of entrainment within the CTD package was observed and flagged suspect on a number of casts. The magnitude of the effect of entrainment by the CTD package is not as significant as has been observed on other cruises. The secondary salinity channel was used to aid screening of the primary channel only and the entrainment features have not been flagged for this channel. Surface features maybe resulting from not soaking the CTD unit for long enough prior to the cast on casts 21s (primary), 26s (secondary), 33s (both channels), 38s (both channels), 43t (both channels), 64s (both channels), 66s (primary) were flagged suspect.

Dissolved oxygen There was some variability flagged in the surface for a number of casts (21s, 25s, 33s, 38s, 43t, 60s and 66s) and lower down the cast profile for another 3 casts; 4t (100-120 db), 22t (90-100 db) and 64s (20-100 db).

Chlorophyll - Fluorometer Cast 1s was very noisy but this noise is not apparent in the rest of the cast profiles. There were high surface values for a number of casts (21s, 23s, 25s, 29s, 48s, 54s, 58s, 60s, 62s, 64s and 66s) and high mid-cast values in other casts (48s, 50s, 52s, 54s, 56s, 58s, 60s, 62s, 64s and 66s).

BBRTD voltages The first cast (1s) displayed great variability than others for the surface part of the cast (0 - 60 db). After cast 6s there were no data returned and the channel has been flagged null.

Turbidity Cast 18t was very noisy and there appeared to be a consistent drop in values over 900-1200 db of the profile. The data below 1400 db for cast 32t were flagged null. Cast 43t showed great variability between 900 - 1800 db and 4600 - 5200 db which were flagged suspect.

Attenuance Data only present for stainless steel casts. Data were only flagged where null or suspect for obvious outliers. The first cast (1s) displayed greater variability than others for the surface part of the cast (0-60 db).

Up and downwelling PAR No sensor on the stainless steel rig and no data returned for casts 2, 4, 5, 18, 32 and 43 using the titanium rig, all data from these casts flagged null. Some casts showed greater variability in the downwelling PAR at the surface, these were not flagged as it is unclear what effect local cloud conditions may have had on the data. The upwelling radiance data for casts 55t, 57t, 61t, 63t, 65t and 67t were also flagged suspect at the top of the cast due to greater variability towards the surface.

Banking

Once quality control screening was complete, the CTD downcasts were banked. Finally, the data were binned against pressure at 1 dbar increments with flagged data excluded from the bin averaging. The primary salinity, temperature, density and potential temperature channels were retained as the best quality data channels from the two sensors.

Field Calibrations

Temperature

No reversing thermometer data were available for AMT16, so the CTD sensor data have not been calibrated against another dataset. Temperature readings from the two temperature sensors were almost identical and no other independent measurements of better quality were available. No further correction was therefore applied to the data.

Salinity

Salinity sensors were calibrated using CTD bottle samples, which were analysed on a bench salinometer (data provided by UKORS). The salinometer data were compared with CTD values from the primary sensor of the stainless steel and titanium casts on the upcast at the time of bottle firing. Analysis showed an offset for the primary sensors on each CTD rig and the stainless rig secondary sensor with a linear offset for the titanium secondary sensor. The calibration offsets were applied through the BODC calibration form.

Casts

Calibration

N

R 2

BODC cal ref

Stainless steel

PSALCC01 = PSALCU01 + 0.0050 (± 0.0813)

59

-

5454

Stainless steel

PSALCC02 = PSALCU02 + 0.0019 (± 0.0747)

59

-

6452

Titanium

PSALCC01 = PSALCU01 + 0.0010 (± 0.0467)

43

-

6377

Titanium

PSALCC02 = 0.9958 * PSALCU02 + 0.1524

46

0.1709

6453

Dissolved oxygen

The oxygen sensors from each rig were calibrated using dissolved oxygen data measured with Winkler titration from discrete bottle samples compared to the measurements from the sensor on each rig from the up cast at the time of bottle firing. Analysis showed a linear relationship with the offset for each CTD rig type and the calibration equations were applied through the BODC calibration form.

Casts

Calibration

N

R 2

BODC cal ref

Stainless steel

DOXYSC01 = 1.1752 * DOXYSU01 - 6.5723

189

0.7317

5536

Titanium

DOXYSC01 = 1.2867 * DOXYSU01 + 7.1566

140

0.8646

5537

Fluorescence

The nominal chlorophyll-a values have been calculated from the fluorometer data (with manufacturer's calibration applied) from the up-cast at bottle firing and the fluorometric chlorophyll-a data from sampled bottles. The calibrations were split between the two fluorometers used on the different CTD rigs. Where samples were not supplied or too few to generate a calibration and could not be grouped with other casts, the fluorometer profiles have not been calibrated. The sampling strategy for the extracted chlorophyll-a dataset used to calibrate the fluorometer focused on the upper water column, therefore the calibration is biased towards these depths. The calibration may not be as reliable below depths ~150m. Casts 1, 2, 61, 63, 64t 65 and 67 have not been calibrated. The extracted chlorophyll-a dataset is available for users to derive their own calibrations should they wish.

The Atlantic Meridional Transect - Phase 2 (2002-2006)

Who was involved in the project?

The Atlantic Meridional Transect Phase 2 was designed by and implemented by a number of UK research centres and universities. The programme was hosted by Plymouth Marine Laboratory in collaboration with the National Oceanography Centre, Southampton. The universities involved were:

University of Liverpool

University of Newcastle

University of Plymouth

University of Southampton

University of East Anglia

What was the project about?

AMT began in 1995, with scientific aims to assess mesoscale to basin scale phytoplankton processes, the functional interpretation of bio-optical signatures and the seasonal, regional and latitudinal variations in mesozooplankton dynamics. In 2002, when the programme restarted, the scientific aims were broadened to address a suite of cross-disciplinary questions concerning ocean plankton ecology and biogeochemistry and the links to atmospheric processes.

The objectives included the determination of:

how the structure, functional properties and trophic status of the major planktonic ecosystems vary in space and time

how physical processes control the rates of nutrient supply to the planktonic ecosystem

how atmosphere-ocean exchange and photo-degradation influence the formation and fate of organic matter

The data were collected with the aim of being distributed for use in the development of models to describe the interactions between the global climate system and ocean biogeochemistry.

When was the project active?

The second phase of funding allowed the project to continue for the period 2002 to 2006 and consisted of six research cruises. The first phase of the AMT programme ran from 1995 to 2000.

Brief summary of the project fieldwork/data

The fieldwork on the first three cruises was carried out along transects from the UK to the Falkland Islands in September and from the Falkland Islands to the UK in April. The last three cruises followed a cruise track between the UK and South Africa, only deviating from the traditional transect in the southern hemisphere. During this phase the research cruises sampled further into the centre of the North and South Atlantic Ocean and also along the north-west coast of Africa where upwelled nutrient rich water is known to provide a significant source of climatically important gases.